President Francois Hollande has asked Manuel Valls, the interior minister to draw up plans to crack down on the “jihadist exodus.” The warning came after it emerged that two 15 year-old French boys had left this month to fight in Syria. They are believed to be among the youngest Westerners to travel there since the Arab Spring revolt against Bashar al-Assad turned into a full blown civil war.

The prospect of hundreds of battle-hardened extremists returning to Europe with training in bomb-making and weaponry has become a major cause of concern in recent months. Mr Valls said the numbers going there had “accelerated” in recent weeks.

He said French intelligence services believe around 700 French nationals or persons living in France were either currently fighting there, had already returned or were planning to become jihadists.

Twenty-one French nationals had been killed in fighting in the Arab state.

The threat is not confined to Europe, Radicalized young Muslims from the US, Canada and Australia as well as from other Muslim countries have traveled to Syria to fight.